PRELIMINARY WRENCH FAULT TECTONIC MAP, EASTERN MONTANA

Four northwest-trending, sinistral, wrench fault zones are interpreted to be present in eastern Montana; Nye Bowler fault zone (NBFZ), Lake Basin fault zone (LBFZ), Willow Creek fault zone (WCFZ), and Cat Creek fault zone (CCFZ). These structures were most recently active during the late Laramide orogeny (Eocene), when northeast to southwest contraction dominated the area, producing left-lateral slip and associated deformation. Slip on these basement-seated faults has created characteristic simple-shear wrench deformation in the sedimentary cover rocks across eastern Montana and into northeastern Wyoming.

The Preliminary Wrench Fault Tectonic Map (PWFTM) of Eastern Montana presents a simple-shear interpretation for surface structures related to sinistral wrench deformation on the NBFZ, LBFZ, WCFZ, and CCFZ. The approximate location of the buried master fault zone(s) is inferred based on orientation of surface structures as related to a simple-shear model used to identify subsidiary wrench structures developed in the sedimentary cover rocks. The simple-shear model thus allows for the uncomplicated definition of wrench structures as parafolds and subsidiary faults (R, R’, P, normal, and reverse) on the PWFTM. Uplifts (e.g., Pryor Mountains and Big Snowy Mountains), basins (e.g., Bull Mountains basin and Crazy Mountains basin), and Cretaceous and Tertiary volcanic terranes (e.g., Sliderock Mountain and Crazy Mountains) may be explained by localized transpression and transtension (e.g., the presence of restraining and releasing bends situated on the master fault(s), pull-apart basins, etc.), rather than calling on more complicated interpretations invoking non-wrench related mechanisms to explain these features. These structures have significant and fundamental implications related to Proterozoic deformation of the Wyoming Archean craton and subsequent Phanerozoic structural and tectonic evolution of the central and northern Rocky Mountains during the Laramide orogeny.